Q: Why can’t you have an atom made entirely out of neutrons?

If you’ve taken a little chemistry you probably know that the electrons in an atom “stack up” in energy levels. The more electrons you add, the higher the electrons have to stack. The same is true for the protons and neutrons inside the nucleus of the atom. What’s a little surprising is that the stack for the protons and the stack for the neutrons are independent of each other.

In a stable atom the energy in the proton stack will be about the same as the energy in the neutron stack. If they’re unequal, then a neutron will turn into a proton ( decay), or a proton will turn into a neutron ( decay) to even out the levels. The greater the difference the sooner the decay, and the more radioactive the atom. There are plenty of exceptions (e.g., Uranium 237 vs. 238), but the pattern usually holds.

Carbon 14 is radioactive because it has too many neutrons. Neutronium has the same problem.

An atomic nucleus made entirely out of neutrons (known to some sci-fi aficionados as “neutronium”) would be completely imbalanced and would decay instantly. It would be tremendously radioactive.

Chemistry nerds may have noticed that heavier elements have more neutrons than protons. For example, Iron 58 has 26 protons, 32 neutrons, and is stable. Forcing protons together takes a lot of energy (likes repel), so after hydrogen, proton energy levels get higher, quicker, than than neutron energy levels.

Exception! In neutron stars you have the added component of lots of gravity. When a proton and an electron fuse into a neutron they take up less room, and since gravity wants to crush everything together, this is a lower energy state. So neutron stars are the one example of stable neutronium. However, most people would say that calling an ex-star an atomic nucleus is pushing the definition of “atomic” a bit far. Even more exciting; neutron stars may also contain the only naturally occurring stable lambda particles!

This brings up a question: Given that protons have an incredibly long lifetime (potentially longer than the so-far age of the universe, according to Leonard Susskind, and that their partners in nucleon-hood have a wee short lifetime of about 15 minutes, how are any atoms stable? Every 15 minutes a proton could have to make up for a neutron loss and become a neutron (…”If they’re unequal, then a neutron will turn into a proton , or a proton will turn into a neutron to even out the levels.”) An atom could become that of an altogether different element roughly every 15 minutes. As this clearly doesn’t happen, what am I missing?

Perhaps my original premise was mistaken or unclear. I had read in a number of places that the average lifespan of a neutron is about fifteen minutes. If this is wrong, my question is meaningless. Are you saying that in a stable atom the average lifespan of a neutron is about as long as that of a proton? Thanks for your patience.

You’re very clear!
The half life of an isolated neutron is about 15 minutes, but that changes with the neutron’s circumstances. Specifically, being in an atom means that the neutron’s half life can be made much shorter, or much, much longer.
In the context of this post, you can think of a neutron as an atom with 1 neutron and 0 protons. By decaying into a proton, the proton and neutron stacks come slightly closer to being in balance.
In a stable atom the neutrons are perfectly happy to exist for an arbitrarily long time.

would it be possible to find a certain number of neutrons that would crate a stable neutronium material, if you didn’t add energy for them to become protons? this material should have incredible properties, it should be extremely dense, as an atom’s size is 10^11 metres across, and a nucleus’ size is 10^15 metres across, so an atom is 10,000 times wider than a neutron. this means you could fit 10^12(1,000,000,000,000) neutrons into the space of an atom. this has crazy implications. this material in the space of a 1kg weight would weigh 10^12 kg. also if you were to compress the earth using this(not that you practically could) it would go from 6,371,000 metres across, to only 637.1 metres across. it would probably behave like an extremely fine dust, so it would probably have to be contained in something. if you fired it through the air, it would have a lot of momentum and inertia, but almost no air resistance.